Interactions among chronic and acute impacts on coral recruits: the importance of size-escape thresholds

Newly settled recruits typically suffer high mortality from disturbances, but rapid growth reduces their mortality once size-escape thresholds are attained. Ocean acidification (OA) reduces the growth of recruiting benthic invertebrates, yet no direct effects on survivorship have been demonstrated. We tested whether the reduced growth of coral recruits caused by OA would increase their mortality by prolonging their vulnerability to an acute disturbance: fish herbivory on surrounding algal turf. After two months' growth in ambient or elevated CO2 levels, the linear extension and calcification of coral (Acropora millepora) recruits decreased as CO2 partial pressure (pCO2) increased. When recruits were subjected to incidental fish grazing, their mortality was inversely size dependent. However, we also found an additive effect of pCO2 such that recruit mortality was higher under elevated pCO2 irrespective of size. Compared to ambient conditions, coral recruits needed to double their size at the highest pCO2 to escape incidental grazing mortality. This general trend was observed with three groups of predators (blenny, surgeonfish, and parrotfish), although the magnitude of the fish treatment varied among species. Our study demonstrates the importance of size-escape thresholds in early recruit survival and how OA can shift these thresholds, potentially intensifying population bottlenecks in benthic invertebrate recruitment.新近定居的幼体通常会因各类扰动面临较高死亡率，但一旦达到体型逃逸阈值，快速生长即可降低其死亡率。海洋酸化（Ocean Acidification, OA）会减缓底栖无脊椎动物幼体的生长速率，但目前尚未证实其对存活率存在直接影响。本研究旨在验证：海洋酸化引发的珊瑚幼体生长减缓，是否会通过延长其对急性扰动的易感性（即草食性鱼类对周边藻垫的啃食），进而提升幼体死亡率。在环境CO₂或升高CO₂浓度条件下培养两个月后，微孔鹿角珊瑚（Acropora millepora）幼体的线性延伸率与钙化速率随CO₂分压（pCO₂）升高而降低。当幼体受到偶然的鱼类啃食时，其死亡率与体型呈负相关关系。但本研究同时发现CO₂分压存在附加效应：无论幼体体型大小，升高的CO₂分压都会导致幼体死亡率上升。与环境CO₂条件相比，在最高CO₂分压下，珊瑚幼体需要达到两倍体型才能逃脱偶然啃食导致的死亡。这一总体趋势在三类捕食者（鳚鱼、刺尾鱼与鹦嘴鱼）的实验中均得到验证，尽管鱼类处理组的效应强度因物种不同存在差异。本研究证实了体型逃逸阈值在幼体早期存活中的重要性，以及海洋酸化如何改变这些阈值，进而可能加剧底栖无脊椎动物招募过程中的种群瓶颈效应。